pgenv: run once scripts

Today pgenv got a new release that provides a simple, but quite useful, feature: the capability to run a custom script the first time the instance is started. The idea is simple: after the initdb phase, if the user has configured a PGENV_SCRIPT_FIRSTSTART executable, the system will run such script against the (just) started instance. This is different from PGENVE_SCRIPT_POSTSTART script, since the latter ie executed every time the cluster has started, while PGENV_SCRIPT_FIRSTSTART is run only the first time the database cluster is started. The aim of this script is, hence, to install users and databases, or populate some initial data.